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From:   David Laight <David.Laight@ACULAB.COM>
To:     'Will Deacon' <will.deacon@arm.com>,
        Mark Salter <msalter@redhat.com>
CC:     Marc Zyngier <marc.zyngier@arm.com>, Khuong Dinh <kdinh@apm.com>,
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        "christoffer.dall@linaro.org" <christoffer.dall@linaro.org>,
        "patches@apm.com" <patches@apm.com>
Subject: RE: [PATCH] arm64: turn off xgene branch prediction while in kernel
 space
Thread-Topic: [PATCH] arm64: turn off xgene branch prediction while in kernel
 space
Thread-Index: AQHTlTKUZVrNGkpW6UiZL00ACo1uxaOEiUrQ
Date:   Thu, 25 Jan 2018 12:44:34 +0000
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From: Will Deacon
> Sent: 24 January 2018 16:43
> On Wed, Jan 24, 2018 at 11:35:03AM -0500, Mark Salter wrote:
> > On Wed, 2018-01-24 at 10:58 +0000, Marc Zyngier wrote:
> > > Khuong,
> > >
> > > On 24/01/18 02:13, Khuong Dinh wrote:
> > > > Aliasing attacks against CPU branch predictors can allow an attacker to
> > > > redirect speculative control flow on some CPUs and potentially divulge
> > > > information from one context to another.
> > > >
> > > > This patch only supports for XGene processors.
...
> > > Why isn't this using the infrastructure that is already in place?
> >
> > That infrastructure relies on a cpu-specific flush of the branch
> > predictor. XGene does not have the ability to flush the branch
> > predictor. It can only turn it on or off.
> 
> So how does this patch protect one user application from another? Sounds
> like you need to turn the thing off at boot and leave it that way, or find
> a sequence of branch instructions to effectively do the invalidation.

What sort of performance penalty does this give?
I can imagine it is significant.

Attempting to flush a branch predictor is also likely to be very slow.

	David